As the development of conventional gas accumulations has matured, less conventional sources are gaining in importance. Globally, volcanic rocks have the potential to host significant gas accumulations that have been historically overlooked. For example, gas has been encountered in volcanic rocks of the YingCheng Group in the SongLiao Basin in the vicinity of the Daqing Oilfield in China as early as the 1970's. However, due to both the difficult drilling and production environment and the challenging reservoir geology, exploitation of these reserves had not been pursued upon their initial discovery.
Volcanic reservoirs are very complex and present problematic features of the two main classes of reservoir-siliciclastics and carbonates-making for a very challenging interpretation environment. They are also relatively under-studied and there is almost no standard methodology for formation evaluation in such environments. Volcanic rocks are variable in terms of composition, with mineralogy much more complex and varied than that commonly encountered in sedimentary rocks. The complex and variable mineralogy makes the determination of matrix properties (e.g., matrix density) and hence porosity very challenging. Further if the reservoir is also hydrocarbon gas, rather than oil bearing, porosity estimation is even more complicated due to a detrimental influence from the presence of gas on most measurements used in porosity computation.
Elemental concentrations in an underground formation may be determined by irradiating the formation with neutrons, detecting the gamma ray spectrum arising from neutron capture by the formation and analyzing the spectrum to determine elemental concentrations. However, due to the complex and variable mineralogy and chemical composition in a volcanic reservoir formation, this prior method has not been applicable in evaluating the volcanic reservoir formation.
While the rock matrix of these volcanic reservoirs is similar to complex siliciclastics (e.g., sandstone or shale), the pore network may have more in common with carbonate reservoirs. Depending on the type of volcanic rock present, pore systems will be dominantly microporous, however zones of brecciation, weathering, and leaching can show a full spectrum of pore types ranging from microporous through macroporous. In massive volcanic rocks, fractures may represent the only porosity.
The complexity of the pore network geometry may render estimation of permeability from wireline log data difficult. The process of determining saturation from resistivity measurements in these reservoirs is challenging. On the one hand, the complex pore network geometry described above raises large uncertainty in terms of the appropriate Archie parameter values to be used, and their variability. On the other hand, diagenesis of the complex mineral assemblage originally forming these rocks has lead to the formation of various clay minerals or other mineral species such as zeolites; all of these contribute to excess conductivity effects that suppress resistivity response to hydrocarbons, analogous to the classical shaley sand problem.